1. Field of the Invention
The invention relates to a polyamide which is degradation-stable on remelting and produced by the anionic polymerisation of lactam. Furthermore, the invention relates to the production of the polyamide using an anionic catalyst system, preferably in a continuous process. The procedure thereby is such that the polymerisation proceeds in a defined processing zone until the conversion of lactam into polylactam has just been effected, after that the activity of the catalyst is neutralized by homogeneous mixing-in of protic compounds, after which further process steps can follow. As a result, a polyamide moulding compound is successfully produced which is stable on reprocessing.
The invention relates furthermore to a method for reprocessing anionically produced polyamide, in which polyamide is crushed and then mixed with the deactivator, melted and extruded.
2. Discussion of Related Art
Anionic lactam polymerisation is comprehensively described in Plastic Material Handbook 3/4, xe2x80x9cPolyamidesxe2x80x9d, Carl Hanser Press, Munich 1998. In practical application, in particular activated anionic lactam polymerisation has thereby been successful and is used preferably for producing tough, voluminous cast parts (cast polyamide).
The activator group, which has acylated the first lactam molecule, thereby forms the one chain end, after which lactamate effects actual chain growth, with ring-opening and reformation of lactamate.
A multiplicity of patents for activated anionic lactam polymerisation relates to the activators.
A special system has become known from DE 197 15 679 A1. Accordingly, species are contained in a salvation medium which, upon addition to the lactam melt, embrace the activator and the catalyst function, so that only one more component must be added to the lactam melt, which component directly initiates then the accelerated lactam polymerisation.
In S. K. Ha, J. L. White: Continuous Polymerisation and Co-polymerization of Lauryl Lactam in a Modular Corotating Twin Screw Extruder, Intern. Polymer Processing XIII (1998) 2, p. 136-141, tests are described where, with separate addition of catalyst (Na-caprolactamate dissolved in lactam-6) and also activator, the lactam polymerisation is implemented continuously in a twin screw extruder. In particular, method parameters are thereby described and optimised. Experiments for further conversion of the granulates and experiments on the behaviour of parts made of anionic polymerised lactam are not described.
A method is known furthermore from DE-OS 22 41 133 for producing polyamides with a high molecular weight. This method also concerns an anionic polymerisation of lactams in anhydrous medium in the presence of alkaline catalysts. It is proposed therein that one or more compounds are added to the obtained polymer which, in the given method conditions, are more strongly acidic than the amide function of the polymers and the lactams and lead only to a limited acidolysis of the polymer chains or to acidolysis equal to 0. For this purpose, sulphonic acid or sulphonic acid esters are proposed as compounds.
As was shown by means of laboratory tests, this freshly polymerised melt could however only be stabilised inadequately. If in fact the acidic supplements mentioned there, especially in the described molar excess, are used in a commonly used extrusion process, then massive corrosion of the machine parts occurs. Furthermore, it has emerged that when a corresponding extrusion strand is drawn as normal through a water bath for cooling and subsequent granulation, H3O+ ions are formed which effect a chain split during remelting for the thermoplastic shaping. Correspondingly acidically set polyamide moulding bodies thereby proved also to be unsuitable for practical usage because they are susceptible to degradation by the effects of hydrolysis, heat and radiation, which leads in particular to a strong viscosity degradation during remelting.
Further methods for neutralizing the catalyst are disclosed in DE-OS 22 41 132 and also in DE-OS 22 41 131. In the first mentioned publication, open to public inspection, tertiary alcohols are mixed in, in particular t-butyl alcohol and in the last mentioned malonic acid esters.
During examination of the last mentioned methods, it emerged that the weak protic compounds added thereto only slow down the anionic lactam polymerisation but do not interrupt it entirely so that this is also displayed in a constantly slight reduction of the MVR (volume melt index), i.e. a nevertheless slight increase in the molecular weight.
It should however be mentioned specially that in particular conventionally anionically produced, non-neutralized polyamide suffers a massive chain length- or viscosity degradation on remelting. Reference is made to EP 0 905 166 A1 in this respect.
It can therefore be established in summary that no anionically produced polyamide is known from the state of the art which displays only a slight viscosity change on remelting.
Proceeding herefrom, it is therefore the object of the present invention to indicate a polyamide which has good physical properties, in particular with respect to processing stability and which at the same time displays only a xe2x80x9cslightxe2x80x9d viscosity degradation on remelting.